Prior attempts to emulate the effects of vacuum tubes with software-based or digital tube-modeling algorithms have failed to fully capture the characteristics of these distortions and faithfully reproduce the dynamic and “warm” sound associated with tube amplifiers. The effects of the cathode-connected R-C network commonly found in tube amplifier stages have been overly simplified in previous art. By use of a chain of linear filters and distortion blocks, the true non-linear dynamical behavior of tube amplifier stages is lost. Many non-linear transfer functions are described by fixed equations and lack means of adjustment of their shape, linear regions, and clipping characteristics. Furthermore, little progress has been made to simplify the non-linear functions used to distort digital signals in these algorithms to improve their computational efficiency and permit greater numbers of them to run on signal processors. While prior examples to capture the characteristics of tube amplifier stages have been successful on many grounds, they either lack the parametric control, versatility, dynamic character, or computational simplicity of the present invention.
U.S. Pat. No. 4,995,084 to Pritchard (Feb. 19, 1991) relates analog circuits to vacuum tube amplifiers and discloses one of the earliest digital versions that approximate the distortion of these circuits. Clipping is achieved with a basic hard-clipping algorithm and does not address controlling the curvature of the clipping regions parametrically. No attention is given to the dynamic distortion effects of tube amplification stages or the elimination of fold-over noise.
U.S. Pat. No. 6,504,935 to Jackson (Jan. 7, 2003) and U.S. Pat. No. 6,611,854 to Amels (Aug. 26, 2003) disclose transfer curves based on trigonometric functions and high-order polynomials which, although allow great versatility in control of harmonic content, take greater efforts to compute. U.S. Pat. No. 5,570,424 to Araya et al. (Oct. 29, 1996), U.S. Pat. No. 5,578,948 to Toyama (Nov. 26, 1996) and U.S. Pat. No. 6,350,943 to Suruga et al. (Feb. 26, 2002) use cubic polynomial functions that are relatively easier to compute but lack a strictly linear region and adjustment of the clipping edge.
U.S. Pat. No. 5,789,689 to Doidic et al. (Aug. 4, 1998) discloses a digital guitar amplifier utilizing several transfer functions to model vacuum tube preamplifier stages. In addition to a hard-clipping function, a fixed curve closely approximating a vacuum tube transfer characteristic is described. However, despite the accuracy of the shape of this model curve, it lacks the parametric control, dynamics, linear regions and computational simplicity of the present invention.
U.S. Pat. No. 4,868,869 to Kramer (Sep. 19, 1989) and U.S. Pat. No. 5,528,532 to Shibutanti (Jun. 18, 1996) are just two of many examples disclosing digital distortion methods implementing non-linear transfer functions using lookup tables located in digital memory. Whereas table lookup methods are extremely computationally efficient, requiring only a single memory read for each processed sample, they don't address or improve the functions with which the tables are filled, nor do they provide means for dynamic or parametric control of the table values. Also, trends for higher sampling resolutions demand lookup tables of impractically large sizes.
U.S. Pat. No. 4,495,640 to Frey (Jan. 22, 1985) recognizes the importance of controlling the gain and offset bias within and between tube amplifier stages for adjustable guitar distortion and implements this in analog circuitry using operational amplifiers between vacuum tube amplifier stages.
U.S. Pat. No. 4,811,401 and U.S. Pat. No. 5,131,044 to Brown et al. (Mar. 7, 1989 and Jul. 14, 1992) demonstrate the need for frequency-dependent control of distortion and highlight, through analog means, the trend for increased forward gain for higher audible frequencies and the high-shelving filter effect. This effect is an inherent property of tube amplifier stages with cathode-connected R-C components. Whereas it is often demonstrated how to simulate this high frequency boost effect with linear filters, the linear filter approach fails to emulate the non-linear dynamical behavior resulting from the feedback effects of the cathode-connected R-C network.
In addition to models of single preamplifier tubes, the value of push-pull amplifier configurations for tube amplifier emulation has been demonstrated in several places in prior art. U.S. Pat. No. 5,321,325 to Lannes (Jun. 14, 1994) discloses a method for adapting a single input to a push-pull configuration which applies to tube- or transistor-based amplifiers aimed at reducing even-order harmonics and accentuating odd-order harmonics. Methods for reproducing crossover distortion by analog means are disclosed in U.S. Pat. No. 3,835,409 to Laub (Sep. 10, 1974), and demonstrate the motivation in the art for reproducing the effects of push-pull power amplifiers for distortion circuits for guitar. U.S. Pat. No. 5,524,055 to Sondermeyer (Jun. 4, 1996) reveals another analog approach to push-pull power amplifier emulation focusing on the power compression effects and soft clipping it produces. None of these examples, however, disclose methods for digitally emulating these effects or for equations to model the transfer functions of power tubes and push-pull power amplifiers.
It has been demonstrated that there is a need in the art for an efficient signal processing method to faithfully reproduce the desired dynamic and distortion effects associated with vacuum tube amplifiers and to provide a means of controlling this emulation via a set of meaningful parameters. The interest to achieve these results has been expressed many times in prior works and has been satisfied by the present invention in an efficient, simple, and readily usable form.